1. Field of the Invention
This invention relates generally to light emitting devices, and more particularly, to producing a self-aligned, self-exposed photoresist pattern on a light emitting diode (LED).
2. Description of Related Art
Semiconductor light-emitting devices such as light emitting diodes are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III-V semiconductors, including binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Light emitting devices based on the III-nitride materials system provide for high brightness, solid-state light sources in the UV-to-yellow spectral regions. Typically, III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Some of these substrates are insulating or poorly conducting. Devices fabricated from semiconductor crystals grown on such substrates must have both the positive and the negative polarity electrical contacts to the epitaxially-grown semiconductor on the same side of the device. In contrast, semiconductor devices grown on conducting substrates can be fabricated such that one electrical contact is formed on the epitaxially grown material and the other electrical contact is formed on the substrate. However, devices fabricated on conducting substrates may also be designed to have both contacts on the same side of the device on which the epitaxial material is grown in a flip-chip geometry so as to improve light extraction from LED chip, to improve the current-carrying capacity of the chip, or to improve the heat-sinking of the LED die. Two types of light emitting devices have the contacts formed on the same side of the device. In the first, called a flip chip, light is extracted through the substrate. In the second, light is extracted through transparent or semi-transparent contacts formed on the epitaxial layers.
Fabrication of an LED requires the growth of an n-type layer or layers overlying a substrate, the growth of an active region overlying the n-type layers, and the growth of a p-type layer or layers overlying the active region. Light is generated by the recombination of electrons and holes within the active region. After fabrication, the LED is typically mounted on a submount. In order to create an LED-based light source that emits white light or some color other than the color of light produced in the active region of the LED, a phosphor is disposed in the path of all or a portion of the light generated in the active region. As used herein, xe2x80x9cphosphorxe2x80x9d refers to any luminescent material which absorbs light of one wavelength and emits light of a different wavelength. For example, in order to produce white light, a blue LED may be coated with a phosphor that produces yellow light. Blue light from the LED mixes with yellow light from the phosphor to produce white light.
One way to produce a phosphor-converted LED is to apply a conformal coating of phosphor over the LED after mounting on the submount. A conformally-coated phosphor-converted LED is described in more detail in application Ser. No. 09/879,547, titled xe2x80x9cPhosphor-Converted Light Emitting Device,xe2x80x9d and incorporated herein by reference. If the conformal coating of phosphor is not uniform, undesirable inconsistencies in the light generated by the phosphor-converted LED can result. Conventionally, an LED was conformally coated by using photo-masking techniques developed for planar semiconductors, where masks are used to define the size and shape of patterns to be printed in photoresist deposited on the LED and submount. The printed photoresist layer defines which areas are covered with phosphor.
The application of conventional masking techniques to three-dimensional structures such as an LED mounted on a submount is fraught with problems including stray reflected light and depth of field artifacts in the resulting image; and imperfect alignment, both of which can result in nonuniform coating of the LED. For example, light reflected from the surfaces of the three dimensional LED structure, including the surface of the photoresist layer used for masking, may introduce exposure artifacts. Also, depth-of-field problems may lead to distortions and loss of dimensional accuracy in the image produced by the mask. Additionally, not all LEDs will have a perfect shape or be perfectly aligned with other LEDs in an array of LEDs. Shape and alignment imperfections can result in nonuniform coating. Masks cannot fully compensate for the process and object variations normally seen in a manufacturing environment, leading to imperfections and yield losses.
In accordance with an embodiment of the invention, a method of forming a photoresist mask on a light emitting device is disclosed. A portion of the light emitting device is coated with photoresist. A portion of the photoresist is exposed by light impinging on the interface of the light emitting device and the photoresist from inside the light emitting device. The photoresist is developed, removing either the exposed photoresist or the unexposed photoresist. In one embodiment, the photoresist mask may be used to form a phosphor coating on the light emitting device. The light emitting device is attached to a submount, and the light emitting device and submount are coated with photoresist. A portion of the photoresist is exposed by light impinging on the interface of the light emitting device and the photoresist from inside the light emitting device. The photoresist is developed to remove the exposed photoresist. A phosphor layer is deposited overlying the light emitting device, then the unexposed portion of photoresist is stripped. In some embodiments, the light exposing the photoresist is produced by electrically biasing the light emitting device, by shining light into the light emitting device through an aperture, or by shining light into the light emitting device by a steered, focussed laser.